The present invention relates generally to a zoom lens system having a short total length and, more specifically, to a high-performance zoom lens system which is of small size and has a high zoom ratio and a large aperture ratio and this lends itself to use in camcorders.
Recent radical reductions in the size, weight and cost of video cameras have increased the popularity of camcorders and their availability to the general public. Typically, a video camera comprises an electric circuit board, an actuator (mechanical) system and an optical system. So far, size and cost reductions have been achieved in the electrical system in particular. More recently, however, some considerable reductions in the size of the image sensor optical system have been pursued. At present, the size and cost reductions of the image sensor optical system are being achieved by the development of a new zoom type which makes effective use of new techniques for miniaturizing imagers, processing aspherical surfaces rotation-symmetrically and automatic TTL focusing. Several examples of those novel zoom lenses are set forth in JP-Kokai-62-24213, JP-Kokai-62-178917, JP-Kokai-62-215225, etc. However, the present need for size and weight reductions, especially, reductions in the total length and the diameter of front lenses are immense. For those reasons, such rear focus versions as set forth in the above-mentioned JP-Kokai-62-24213, 62-178917 and 62-215225 and some versions in which the compensator unit is located in the rear of a stop have now been adopted. These versions may possibly have astonishing latent faculties of reducing the total length and making the front lens diameter small. Referring especially to JP-Kokai-62-178917, it teaches that some considerable reduction in the number of constituent lenses is achievable by applying an aspherical surface to the above-mentioned image formation system and sufficient correction of aberrations is attainable as well. However, virtually no size reduction can be achieved because the total length and front lens diameter are not substantially different from those of conventional or classical lens configuration.
That is to say, the system disclosed in JP-Kokai-62-178917 comprises a zoom subsystem including a first unit having a positive refracting power and a second unit having a negative refracting power and an image formation subsystem including a third unit consisting solely of a positive single lens having an aspherical surface and always remaining fixed and a fourth unit having at least one negative lens or consisting generally of two or three lenses and movable during zooming and for regulating the focal position depending on how far the subject is spaced. Thus, the use of the rear focus which also serves as a compensator and the aspherical surface renders it possible to reduce the number of constituent lens to ten or below, thereby reducing extra space. This thus enables the front lens diameter to be reduced and makes it possible to reduce the total length. The rear focus configuration could make it easy to increase the power of the first unit. However, this is not the case and the power of the second unit remains weak as well. In addition, using a single lens for the third unit incurs another disadvantage. In other words, the luminous flux cannot be converged well here, so that it can not afocally exit. For this reason, nothing remains but to increase the focal length of the fourth unit, and the back focus remains long as well; that is, no sufficient reductions in the total length, front lens diameter, etc. are achieved.